Processes for forming an aldehyde by the reaction of an olefin with carbon monoxide and hydrogen have been known as hydroformylation processes or oxo processes. For many years, all commercial hydroformylation reactions employed cobalt carbonyl catalysts which required relatively high pressures (often on the order of 100 atmospheres or higher) to maintain catalyst stability.
U.S. Pat. No. 3,527,809, issued Sept. 8, 1970, to R. L. Pruett and J. A. Smith, discloses a significantly new hydroformylation process whereby alpha-olefins are hydroformylated with carbon monoxide and hydrogen to produce aldehydes in high yields at low temperatures and pressures, where the normal to iso-(or branched-chain) aldehyde isomer ratio of the product aldehydes is high. This process employs certain rhodium complex catalysts and operates under defined reaction conditions to accomplish the olefin hydroformylation. Since this new process operates at significantly lower pressures than required theretofore in the prior art, substantial advantages were realized including lower initial capital investment and lower operating costs. Further, the more desirable straight-chain aldehyde isomer could be produced in high yields.
The hydroformylation process set forth in the Pruett and Smith patent noted above includes the following essential reaction conditions:
(1) A rhodium complex catalyst which is a complex combination of rhodium with carbon monoxide and a triorganophosphorus ligand. The term "complex" means a coordination compound formed by the union of one or more electronically rich molecules or atoms capable of independent existence with one or more electronically poor molecules or atoms, each of which is also capable of independent existence. Triorganophosphorus ligands whose phosphorus atom has one available or unshared pair of electrons are capable of forming a coordinate bond with rhodium.
(2) An alpha-olefin feed of alpha-olefinic compounds characterized by a terminal ethylenic carbon-to-carbon bond such as a vinyl group CH.sub.2 .dbd.CH--. They may be straight chain or branched chain and may contain groups or substituents which do not essentially interfere with the hydroformylation reaction, and they may also contain more than one ethylenic bond. Propylene is an example of a preferred alpha-olefin.
(3) A triorganophosphorus ligand such as a triarylphosphine. Desirably each organo moiety in the ligand does not exceed 18 carbon atoms. The triarylphosphines are the preferred ligands, an example of which is triphenylphosphine.
(4) A concentration of the triorganophosphorus ligand in the reaction mixture which is sufficient to provide at least two, and preferably at least 5, moles of free ligand per mole of rhodium metal, over and above the ligand complexed with or tied to the rhodium atom.
(5) A temperature of from about 50.degree. to about 145.degree. C., preferably from about 60.degree. to about 125.degree. C.
(6) A total hydrogen and carbon monoxide pressure which is less than 450 pounds per square inch absolute (psia), preferably less than 350 psia.
(7) A maximum partial pressure exerted by carbon monoxide no greater than about 75 percent based on the total pressure of carbon monoxide and hydrogen, preferably less than 50 percent of this total gas pressure.
It is known that, under hydroformylation conditions, some of the product aldehydes may condense to form by-product, high boiling aldehyde condensation products such as aldehyde dimers or trimers U.S. Pat. No. 4,148,830 discloses the use of these high boiling liquid aldehyde condensation products as a reaction solvent for the catalyst. In this process, solvent removal from the catalyst, which may cause catalyst losses, is unnecessary and, in fact, a liquid recycle containing the solvent high boiling aldehyde condensation products and catalyst is fed to the reaction zone from a product recovery zone. It may be necessary to remove a small purge stream to prevent the buildup of such aldehyde condensation products and poisons to the reaction to excessive levels of concentration.
More specifically, as pointed out in said U.S. Pat. No. 4,148,830, some of the aldehyde product is involved in various reactions as depicted below using n-butyraldehyde as an illustration: ##STR1##
The names is parentheses in the afore-illustrated equations, aldol I, substituted acrolein II, trimer III, trimer IV, dimer V, tetramer VI, and tetramer VII, are for convenience only. Aldol I is formed by an aldol condensation; trimer III and tetramer VII are formed via Tischenko reactions; trimer IV by a transesterification reaction; dimer V and tetramer VI by a dismutation reaction. Principal condensation products are trimer III, trimer IV, and tetramer VII, with lesser amounts of the other products being present. Such condensation products, therefore, contain substantial quantities of hydroxylic compounds as witnessed, for example, by trimers III and IV and tetramer VII.
Similar condensation products are produced by self-condensation of iso-butyraldehyde and a further range of compounds is formed by condensation of one molecule of normal butyraldehyde with one molecule of iso-butyraldehyde. Since a molecule of normal butyraldehyde can aldolize by reaction with a molecule of iso-butyraldehyde in two different ways to form two different aldols VIII and IX, a total of four possible aldols can be produced by condensation reactions of a normal/iso mixture of butyraldehydes. ##STR2##
Aldol I can undergo further condensation with isobutyraldehyde to form a trimer isomeric with trimer III and aldols VIII and IX and the corresponding aldol X produced by self-condensation of two molecules of isobutyraldehyde can undergo further reaction with either normal or isobutyraldehyde to form corresponding isomeric trimers. These trimers can react further analogously to trimer III so that a complex mixture of condensation products is formed.
Commonly-assigned, copending U.S. application Ser. No. 674,823, filed Apr. 8, 1976, discloses a liquid phase hydroformylation reaction using a rhodium complex catalyst, wherein aldehyde reaction products and some of their higher boiling condensation products are removed in vapor form from the catalyst containing liquid body (or solution) at the reaction temperature and pressure. The aldehyde reaction products and the condensation products are condensed out of the off gas from the reaction vessel in a product recovery zone and the unreacted starting materials (e.g., carbon monoxide, hydrogen and/or alpha-olefin) in the vapor phase from the product recovery zone are recycled to the reaction zone. Furthermore, by recycling gas from the product recovery zone coupled with make-up starting materials to the reaction zone in sufficient amounts, it is possible, using a C.sub.2 to C.sub.5 olefin as the alpha-olefin starting material, to achieve a mass balance in the liquid body in the reactor and thereby remove from the reaction zone at a rate at least as great as their rate of formation essentially all the higher boiling condensation products resulting from self-condensation of the aldehyde product.
More specifically, according to the above latter application, a process for the production of an aldehyde containing from 3 to 6 carbon atoms is disclosed which comprises passing an alpha-olefin containing from 2 to 5 carbon atoms together with hydrogen and carbon monoxide at a prescribed temperature and pressure through a reaction zone containing the rhodium complex catalyst dissolved in a liquid body, continuously removing a vapor phase from the reaction zone, passing the vapor phase to a product separation zone, separating a liquid aldehyde containing product in the product separation zone by condensation from the gaseous unreacted starting materials, and recycling the gaseous unreacted starting materials from the product separation zone to the reaction zone. Preferably, the gaseous unreacted starting materials plus make-up starting materials are recycled at a rate at least as great as that required to maintain a mass balance in the reaction zone.
It is known in the prior art that rhodium hydroformylation catalysts, such as hydrido carbonyl tris(triphenylphosphine)rhodium, are deactivated by certain extrinsic poisons which may be present in any of the gases fed to the reaction mixture. See, for example, G. Falbe, "Carbon Monoxide in Organic Synthesis", Springer-Verleg, New York, 1970. These poisons (X), termed virulent poisons, are derived from materials such as sulfur-containing compounds (e.g., H.sub.2 S, COS, etc.), halogen-containing compounds (e.g., HCl, etc.), cyano-containing compounds (e.g., HCN, etc.), and the like, and can form Rh-X bonds which are not broken under mild hydroformylation conditions. If one removes such poisons from the materials fed to the reaction mixture, to below 1 part per million (ppm), one would expect therefore that no such deactivation of the catalyst would occur. However, it has been found that such is not the case. For example, when very clean gases (&lt;1 ppm extrinsic poisons) were used in the hydroformylation of propylene and the gas recycle technique discussed above was employed, under the following conditions:
______________________________________ temperature (.degree.C.) 100 CO partial pressure (psia) 36 H.sub.2 partial pressure (psia) 75 olefin partial pressure (psia) 40 ligand/rhodium mole ratio 94 ______________________________________
the catalyst activity decreased at a rate of 3% per day (based on the original activity of the fresh catalyst). It appears therefore that even the substantially complete removal of extrinsic piosons does not prevent such catalyst deactivation, which we shall term intrinsic catalyst deactivation.
The prior art, to our knowledge, does not propose a solution to this problem of intrinsic deactivation of rhodium hydroformylation catalysts, nor does it even recognize the reasons for the same.
Japanese Patent Application No. Sho-49-85523 discloses that in a process of hydroformylating olefins using a rhodium-tertiary phosphine catalyst, wherein catalyst-containing solution separated from the reaction product is recycled to the reaction and reused, high-boiling by-products and complexes which do not have any or have a reduced catalytic activity formed by a change in the structure of the rhodium-tertiary phosphine complex itself and by the action of impurities such as oxygen, halogens, sulfur, etc., contained to a small extent in the starting materials, accumulate gradually in the catalyst solution. The patent states that in order to carry out the hydroformylation reaction continuously and in a stable manner, the catalytic activity of the recycle catalyst solution is maintained at a constant level by supplying fresh catalyst to the recycled solution and at the same time removing a part of the recycled catalyst solution. With this procedure, the rhodium removed from the catalyst solution must be recovered due to the cost of the rhodium. However, processes for recovering rhodium from solution are complicated and the resulting hydroformylation reaction becomes economically disadvantageous. This patent proposes a process for reactivating the inactivated catalyst by treating the catalyst solution with carbon dioxide.
W. Strohmeier and A. Kuhn, in Journal of Organometallic Chemistry, 110, 265-270 (1976), discussed the hydroformylation of 1-hexene under mild conditions (40.degree. C., 1 atmosphere) using HRhCO[P(C.sub.6 H.sub.5).sub.3 ].sub.3 as a catalyst in the absence of a solvent, and noted a deactivation of the catalyst. The effects of P(C.sub.6 H.sub.5).sub.3 addition and CO partial pressure on the conversion are studied and it is stated that optimum conversion (at a normal:iso aldehyde ratio of 99:1) is obtained with a H.sub.2 :CO ratio of 1:1 and with the addition of P(C.sub.6 H.sub.5).sub.3. The authors do not state the cause of the observed deactivation nor do they propose a solution.
G. Wilkinson and his colleagues noted a deactivation of the catalyst HRhCO[P(C.sub.6 H.sub.5).sub.3 ].sub.3 when used in the hydrogenation of alkenes [M. Yagupsky et al, Journal of the Chemical Society (A), 1970, pages 937-941], and in fact noted that in a hydroformylation process no loss in activity of this catalyst has been noted "even after many cycles" (see page 937), citing C. K. Brown and G. Wilkinson, Tetrahedron Letters, 1969, 1725.
U.S. Pat. No. 3,555,098 discloses a process for avoiding the deactivation of a hydrocarbonylation catalyst by treating all or a portion of a liquid recycle reaction medium with an aqueous solution. It is disclosed that this treatment removes carboxylic acid byproducts (formed by oxidation of the product aldehydes/alcohols) and prevents the deactivation of the reaction medium.
We have found that the aforementioned intrinsic deactivation of rhodium catalysts under hydroformylation conditions is caused by the combination of the effects of temperature, the partial pressures of both carbon monoxide and hydrogen and the phosphine ligand:rhodium mole ratio. It has further been determined that this deactivation produces non-catalytically active material. It would be desirable to minimize or eliminate this intrinsic deactivation problem in order to achieve a truly optimum commercial operation; that is, a rhodium-catalyzed hydroformylation reaction which produces the desired product at commercially attractive conversion rates at conditions such that the catalyst remains active over a prolonged period of time.